Neutrons put together 40 years of puzzles behind the mysterious magnetism of iron iodide

Researcher Xiaojian Bai and his colleagues used neutrons at the ORNL spallation neutron source to discover hidden quantum fluctuations in a simple iron iodide material discovered in 1929. Research suggests that many similar magnetic materials may have quantum properties waiting to be discovered. (Photo credit: ORNL / Genevieve Martin)

Advanced materials with newer properties are almost always developed by adding more elements to the ingredient list. However, quantum research suggests that some simpler materials may already have advanced properties that scientists simply could not see before.

Researchers at Georgia Tech and the University of Tennessee-Knoxville discovered hidden and unexpected quantum behavior in a relatively simple iron iodide (FeI2) material discovered nearly a century ago. The new research on the behavior of the material was made possible by a combination of neutron scattering experiments and theoretical physical calculations at the Oak Ridge National Laboratory (ORNL) of the Department of Energy (DOE).

The team’s results, published in the journal Nature Physics, solve a 40-year-old puzzle surrounding the material’s mysterious behavior and could be used as a map to unlock a treasure trove of quantum phenomena in other materials.

“Our discovery was in large part driven by curiosity,” said Xiaojian Bai, the newspaper’s lead author. Bai received his PhD from Georgia Tech and is a postdoctoral fellow at ORNL, where he uses neutrons to study magnetic materials. “I came across this iron iodide material in 2019 as part of my doctoral thesis. I have tried to find connections using a magnetic triangular grid arrangement that has something called “frustrated magnetism”. “

In conventional magnets such as refrigerator magnets, the electrons in the material are arranged in a line like arrows, either all pointing in the same direction up or down, or alternating between up and down. The directions in which the electrons point are called “spins”. In more complex materials such as iron iodide, however, the electrons are arranged in a triangular lattice in which the magnetic forces between the three magnetic moments conflict and are not sure in which direction to point – hence “frustrated magnetism”.

“As I read through all of the literature, I noticed this compound, iron iodide, which was discovered in 1929 and studied a little intensely in the 1970s and 80s,” Bai said. “At the time, they were seeing some quirks or unconventional behaviors, but they didn’t really have the resources to fully understand why they were seeing it. So we knew there was something unresolved that was strange and interesting, and we have much more powerful experimental tools than we did forty years ago. So we decided to re-examine this problem and hoped to provide some new insight. “

Quantum materials are often described as systems that exhibit exotic behavior and disobey the classical laws of physics – like a solid material that behaves like a liquid, with particles that move like water and refuse to freeze or theirs, even at freezing temperatures Stop movement. Understanding how these exotic phenomena work or their underlying mechanisms is key to advancing electronics and developing other next-generation technologies.

“In quantum materials, two things are of great interest: phases of matter such as liquids, solids and gases and excitations of these phases such as sound waves. Similarly, spin waves are excitations of a magnetic solid material, ”said Martin Mourigal, professor of physics at Georgia Tech. “Our search for quantum materials has long been about finding exotic phases, but the question we have asked ourselves in this research is, ‘Perhaps the phase itself isn’t seemingly exotic, but what if its stimuli are? ‘And that’s exactly what we found. “

A small sample of iron iodide held by Bai (above) is mounted and prepared for neutron scattering experiments, which were used to measure the basic magnetic excitations of the material.  (Photo credit: ORNL / Genevieve Martin)

A small sample of iron iodide held by Bai (above) is mounted and prepared for the neutron scattering experiments that were used

to measure the basic magnetic excitations of the material. (Photo credit: ORNL / Genevieve Martin)

Neutrons are ideal probes for studying magnetism because they themselves act like microscopic magnets and can be used to interact with and excite other magnetic particles without affecting the atomic structure of a material.

Bai was introduced to neutrons as a graduate student from Mourigal at Georgia Tech. Mourigal has been a frequent user of neutron scattering on the High Flux Isotope Reactor (HFIR) and the Spallation Neutron Source (SNS) from ORNL for several years. In doing so, he used the user facilities of the DOE Office of Science to study a wide range of quantum materials and their diverse and bizarre behaviors.

When Bai and Mourigal exposed the iron iodide material to a beam of neutrons, they expected a certain excitation or energy band associated with a magnetic moment of a single electron. instead they saw not one but two different quantum fluctuations radiating simultaneously.

“With neutrons we could see this hidden fluctuation very clearly and measure its entire excitation spectrum, but we still haven’t understood why we are observing such abnormal behavior in what appears to be a classical phase,” said Bai.

For answers, they turned to theoretical physicist Cristian Batista, Lincoln Chair Professor at the University of Tennessee-Knoxville and Deputy Director of the Shull Wollan Center of ORNL, a joint neutron science institute that provides visiting researchers with additional resources and expertise on neutron scattering Provides.

With the help of Batista and his group, the team was able to mathematically model the behavior of the mysterious quantum fluctuation and, after performing additional neutron experiments with the CORELLI and SEQUOIA instruments at SNS, identify the mechanism that caused it to appear.

“What the theory predicted, and what we were able to confirm with neutrons, is that this exotic fluctuation occurs when the spin direction between two electrons is reversed and their magnetic moments flip in opposite directions,” said Batista. “When neutrons interact with the spins of the electrons, the spins rotate synchronously along a certain direction in space. This choreography triggered by neutron scattering generates a spin wave. “

He explained that electronic spins in different materials can take on many different orientations and spin choreographies that create different types of spin waves. In quantum mechanics, this concept is known as “wave-particle duality”, in which the new waves are viewed as new particles and, under normal conditions, typically remain hidden from neutron scattering.

“In a way, we’re looking for dark particles,” added Batista. “We can’t see them, but we know they’re there because we can see their effects or the interactions they have with the particles we can see.”

“In quantum mechanics there is no difference between waves and particles. We understand the behavior of the particle based on wavelength and that’s what we can measure with neutrons, ”said Bai.

Mourigal compared the way neutrons recognize particles to waves breaking around rocks on the surface of the ocean.

“In still water, we can’t see the rocks on the ocean floor until a wave moves over them,” said Mourigal. “Only by generating as many waves as possible with neutrons was Xiaojian able to use Cristian’s theory to identify the rocks or, in this case, the interactions that make the hidden fluctuations visible.

The use of quantum magnetic behavior has already led to technological advances such as the MRI machine and magnetic hard disk storage that catalyzed personal computing. More exotic quantum materials can accelerate the next wave of technology.

In addition to Bai, Mourigal and Batista, the authors of the paper include Shang-Shun Zhang, Zhiling Dun, Hao Zhang, Qing Huang, Haidong Zhou, Matthew Stone, Alexander Kolesnikov and Feng Ye.

Since their discovery, the team has used these findings to develop and test predictions for a wider range of materials that they expect to produce more promising results.

“When we add more ingredients to a material, we also increase potential problems such as disruptions and heterogeneities. If we want to understand and create really clean material-based quantum mechanical systems, it may be more important to fall back on these simple systems than we thought, ”said Mourigal.

“This solves the 40-year-old riddle of the mysterious excitement in iron iodide,” said Bai. “Today we have the advantage that we can further develop large neutron systems such as SNS, with which we can basically examine the entire energy and momentum space of a material in order to see what happens to these exotic stimuli.

“Now that we understand how this exotic behavior works in a relatively simple material, we can imagine what we might find in more complex ones. This new understanding has motivated us and will hopefully motivate the scientific community to study more such materials, which will certainly lead to more interesting physics. “

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